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Image Quality and Stability Criteria

Alex Rea adr34@cam.ac.uk

Cavendish Lab oratory, University of Cambridge

FTT Meeting Friday 7th May 2010

Alex Rea (Cavendish Labs, Cambridge)

Image Quality and Stability Criteria

7th May 2010

1/9


Off-Axis Angle

· MTF reference surfaces · MRTD test systems · Spectrograph mirrors

Quality a

Optical S of making reducing of fixed f from whic the requi adaptabi suit you a smaller s diameter size we m

Mounted

h = off axis distance (OAD) h = 2f(tan /2) g = apparent focal length = off axis angle (OAA)
Alex Rea (Cavendish Labs, Cambridge) Image Quality and Stability Criteria 7th May

Accuracy use of hi Surfaces mounts s use. Corr distortion 2010from / 9 h 2 us


Provisional Layout

Alex Rea (Cavendish Labs, Cambridge)

Image Quality and Stability Criteria

7th May 2010

3/9


Image Quality
Criterion was that spread of the image should be no more than two Airy disks across a field of view of ± 10 arcseconds. Elements were set up correctly, and then perturbed. Element Dichroic Degree of Freedom x, y, z x , y z x, y z x , y z x, y z x , y Allowed movement Insensitive 6 arcmin Unconstrained Unconstrained 600µm 12 arcmin Unconstrained Unconstrained 600µm Insensitive
7th May 2010 4/9

OAP

CCD

Alex Rea (Cavendish Labs, Cambridge)

Image Quality and Stability Criteria


Image Quality - Summary

From a quality perspective, the defocus constraint is easy to meet. The angular requirements require the light reflected from the dichroic and the axis of the parent parabola of the OAP to be parallel; it is only the relative angle between the OAP and the dichroic that concerns us for the image quality. This fact can therefore perhaps be used to our advantage in the design of the mount? There are still a couple of sources of potential error not included here...

Alex Rea (Cavendish Labs, Cambridge)

Image Quality and Stability Criteria

7th May 2010

5/9


Image Quality - Implementation
Manufacturing uncertainties of the parabola can be corrected for, but we need to know what those are. Unfortunately, no indication of the expected uncetainty on the direction of axis. In communication with SORL.

Alex Rea (Cavendish Labs, Cambridge)

Image Quality and Stability Criteria

7th May 2010

6/9


Image Stability - Absolute
Absolute stability requirements are hard - but we already knew this. Criterion was image should not move by more than 0.015 arcseconds, or = 1.2µm for the focal length being considered here. Element Dichroic OAP Degree of Freedom x, y, z x , y x, y z x , y , z x, y z x , y Allowed movement Insensitive 0.36 arcsec 1.2µm Unconstrained arctan = 0.21 arcsec f arctan O = 0.76 arcsec AD 1.2µm Unconstrained Insensitive
7th May 2010 7/9

CCD

Alex Rea (Cavendish Labs, Cambridge)

Image Quality and Stability Criteria


Image Stability - Expansion

Alex Rea (Cavendish Labs, Cambridge)

Image Quality and Stability Criteria

7th May 2010

8/9


Image Stability - Expansion - < 1.2µm
T 0.5 1 2 5 0.5 1 2 5 yD 2.13 4.25 8.50 21.25 0 0 0 0 zD 1.68 3.35 6.70 16.75 3.56 7.12 14.24 35.6 yP 4.14 8.28 16.56 41.4 2.05 4.10 8.20 20.5 zP 0.19 -0.38 -0.76 -1.88 0 0 0 0 yC -0.1775 -0.355 -0.71 -1.78 -0.23 -0.46 -0.92 -2.3 zC 2.40 4.81 9.62 24.05 0.43 0.85 1.70 4.25 Movement 1.53 3.08 6.14 15.4 1.4 2.81 5.58 14.07

Lengths in µm, temperature in C

(Uniform) expansion under the Defocus is

expansion of the table does not affect angles. Top data have the about the centre of the table, bottom data have the expansion light ray. not shown, but is under 50µm in all cases.
Image Quality and Stability Criteria 7th May 2010 9/9

Alex Rea (Cavendish Labs, Cambridge)